This invention relates to a process of preparing high purity silicon tetrafluoride useful for the preparation of amorphous silicon which is expected as an advantageous material for various electronic devices and particularly for photovoltaic cell elements.
From a purely scientific point of view, silicon tetrafluoride can be obtained by reaction between silicon oxide and hydrofluoric acid according to the following equation. EQU SiO.sub.2 +4HF.fwdarw.SiF.sub.4 +2H.sub.2 O (1)
In the precense of water in liquid state, however, silicon tetrafluoride readily undergoes hydrolysis to form hexafluorosilicic acid and silicon oxide, which is in the form of hydrated gel-like silica, as represented by the following equation. EQU 3SiF.sub.4 +2H.sub.2 O.fwdarw.2H.sub.2 SiF.sub.6 +SiO.sub.2 ( 2)
From an industrial viewpoint, the preparation of silicon tetrafluoride by the reaction of Equation (1) becomes intolerably inefficient if this reaction is accompanied by the hydrolysis reaction of Equation (2) because, besides a considerable lowering in the yield of silicon tetrafluoride, piping of the reaction apparatus is frequently and significantly choked with the hydrated silica formed by the hydrolysis. Therefore, it is necessary to suppress the hydrolysis reaction of Equation (2) either by preventing condensation of H.sub.2 O formed in the reaction of Equation (1) or by effectively separating H.sub.2 O from SiF.sub.4 by causing the gaseous reaction product to come into contact with a suitable medium which is absorbent of water.
For example, U.S. Pat. No. 3,674,431 proposes to cause hydrogen fluoride gas to react with a pulverized crystalline silicon oxide such as quartz, cristobalite or tridymite by introducing water in a vapor-liquid mixed phase and hydrogen fluoride gas simultaneously into a packed column of the silicon oxide with heating to keep the temperature of the packed column at 80.degree.-110.degree. C. The reaction product of this method is a mixture of silicon tetrafluoride, hydrogen fluoride and water. However, this method is disadvantageous in the troublesomeness of introducing water in a vapor-liquid mixed phase simultaneously with hydrogen fluoride gas and also in the necessity of heating the packed column to a temperature in the range from 80.degree. to 110.degree. C. U.S. Pat. No. 2,861,872 proposes to suspend a powdered crystalline silicon oxide in a suitable organic medium such as glycerol or ethylene glycol and introduce hydrogen fluoride gas into the suspension with a view to preparing high purity silicon tetrafluoride, but this method is rather unsuited to industrial practice firstly because a considerably high reaction temperature such as 120.degree.-177.degree. C. becomes a requisite and secondly because a considerable loss of the organic medium by evaporation is inevitable. U.S. Pat. No. 2,833,628 teaches to prepare silicon tetrafluoride by dispersing silicon oxide in sulfuric acid and adding 20-28% hexafluorosilicic acid to the sulfuric acid slurry. In this process it is desirable that the concentration of sulfuric acid be above 65% because the amount of silicon tetrafluoride remaining dissolved in the sulfuric acid slurry sharply increases when the concentration of sulfuric acid becomes below 65%. This method too is rather unfavorable to industrial practice because of the need of using a large quantity of sulfuric acid to absorb a large amount of water contained in the relatively low concentration hexafluorosilicic acid and the consequential need of using a reaction vessel of an unfavorably large capacity.